This invention relates to a method and its apparatus for reviewing defects on a sample by using image acquisition means.
To improve production yield in the manufacture of semiconductor devices, it has been of importance to quickly clarify the cause of occurrence of defects in the manufacturing process. A defect inspection apparatus and a defect reviewing apparatus have now been used in combination at present in the actual scene of the manufacture to analyze the defects. The defect inspection apparatus is the one that inspects semiconductor wafer to detect defects by using optical means or electron beams and outputs feature quantities of the defects detected and their coordinate information.
It is important for the defect inspection apparatus to inspect at a high speed a broad range on a semiconductor wafer. Therefore, the pixel sizes of the images to be acquired have been increased as much as possible (to result in lower resolution) so as to reduce the image data quantity and to keep high through-put. Though the approximate feature of the defect can be grasped by confirming the existence of the defect from the detected images of low resolution, the kind of the defect cannot be discriminated in detail in most cases. The reviewing apparatus is therefore employed to discriminate in detail the kind of the defects by acquiring the images of defects having relatively high resolution. The reviewing apparatus is the apparatus that images the image of the defect on the semiconductor wafer by using the output of the inspection apparatus and outputs the image of the defect having high resolution.
The defect size has often reached the order of dozens of nm (nanometers) with the progress of miniaturization in the semiconductor manufacturing process and resolution of the order of several nm is necessary to review in detail the defects. Therefore, a reviewing apparatus using a scanning electron microscope (hereinafter called “review SEM”) has gained a wide application in recent years. Automation of the inspection operation has been desired in the mass-production line of semiconductors and the review SEM has an ADR (Automatic Defect Review) function of automatically collecting images of defect coordinates inside a wafer and an ADC (Automatic Defect Classification) function of automatically classifying the images acquired.
The ADR function is the one that automatically collects the images taken in high magnification of defect portions represented by the defect coordinates by using the defect inspection apparatus. The problem that arises hereby is an error between the defect coordinates outputted by the defect inspection apparatus and actual defect coordinates. Because variance of about ±4 [μm] generally exists as the error, there is the possibility that the defect does not enter the visual field when the defect coordinates outputted by the defect inspection apparatus are taken in high magnification (such as 50,000 times) having a visual field of about 2.5 [μm]. Therefore, imaging is made first in low magnification (such as 15,000 times) of a visual field of about 9 [μm]. The defect position is then specified from this low magnification image and finally, the defect coordinates specified on the image of the low magnification are taken in high magnification (such as about 50,000 times) to acquire the high magnification image of the defect.
One of the known methods for specifying the defect position is a comparison inspection that compares a defect image of a defect portion taken in low magnification with a reference image obtained by imaging a portion at which the same pattern as the defect portion is formed and which is taken in low magnification, and detects the difference of both images as a defect. Since a plurality of the same chips are arranged on a semiconductor wafer, it is possible to use, as a reference image, the image of a position spaced apart by one chip from the coordinates at which the defect exists and taken in low magnification. When the same wiring pattern is periodically formed as in a cell portion of a memory, the image of the same wiring pattern can be used as the reference image.
As an aperture ratio of semiconductor wafers has become greater in recent years, the number of defects to be reviewed per wafer has increased, too. In addition, since through-put of the review apparatus is lower than that of the inspection apparatus, the ADR operation must be sped up.
A long time is generally necessary in the ADR operation in the stage movement from an initial position to a target position and in imaging of both reference and defect images. To speed up the ADR operation, a method that skips several of the imaging procedures will be effective besides speed-up of these process steps. The procedure to be skipped is generally imaging of the reference image. Several methods have been proposed such as a method that prepares in advance a reference image and a method that synthesizes the reference image from the defect image and conducts comparison inspection.
The former method is described in JP-A-2000-67243. This method stores in advance a periodic pattern as a reference image, conducts the comparison inspection by using this image and the defect image and specifies a defect position.
The latter is described in JP-A-2003-98114. The method compares local regions having similar appearances on a defect image, calculates reliability of the defect detection of their difference region on the basis of probability distribution of the normal portion and detects a difference region having high reliability as the defect. Another method is described in JP-A-2007-40910. The method synthesizes a reference image by utilizing repetition periodicity of circuit patterns imaged in the defect image and detects the defect through the comparison inspection with the reference image so synthesized.
As a method for conducting the comparison inspection by using the defect image and the reference image, JP-A-2001-325595 discloses a method that executes a binarization processing for the difference image between the two images and detects the defect portion.
When the method of JP-A-2007-40910 for synthesizing the reference image by utilizing the periodicity of the circuit pattern contained in the defect image from the defect image acquired by imaging the region containing the defect is used, the reference image cannot be synthesized correctly in a region not having periodicity and existing at a part inside the image (hereinafter called “peculiar portion”). The explanation will be given hereby by way of example about the case where the reviewing method of JP-A-2007-40910 is applied to the low magnification defect image 701 containing a defect 705 and a peculiar portion 702 inside an image shown in FIG. 7.
A reference numeral 704 is synthesized in S703 by utilizing the periodicity of the patterns contained in the low magnification defect image 701 acquired by imaging with SEM and a difference image between the defect image 701 and the reference image 704 synthesized is determined as a comparison inspection in S706. A binary difference image 707 is generated by binarizing this difference image and a defect 709 is detected from this binary difference image 707. Not only a defect portion 709 but also a peculiar portion 708 is detected from the binary difference image 707 acquired by binarizing the difference image.
To prevent the peculiar portion 708 from being detected wrongly as the defect, the invention of JP-A-2007-40910 regards the difference portion acquired by the comparison inspection as a defect applicant, calculates a feature quantity for each defect applicant and detects only a real defect by identifying the feature quantities.
The peculiar portion of the circuit pattern represented by reference numeral 702 in FIG. 7 is believed to have a variety of features resulting from the difference of production processes of semiconductors. To improve a correct solution ratio of judgment in the judgment method using the feature quantities, greater feature quantities must be calculated from the image. However, this results in the increase of the processing time of the defect detection processing because a long processing time is generally necessary for calculating feature quantities from images. When unimaginable peculiar portions exist, the peculiar portions may most possibly be detected as the defect because the method can correctly judge only the peculiar portions having the features that are assumed in advance.